1. Field of the Invention
The present invention relates to a variable output-type constant current source circuit capable of varying its output current in a stepwise manner, and more particularly to a technology for allowing an output current value of a variable output-type constant current source circuit to be sufficiently varied.
2. Description of the Prior Art
An integrated circuit incorporating various electronic circuits in high-density packaging often has the need for supplying a stable current to each of the electronic circuits to prevent a supply voltage and other disturbances from adversely affecting on the operation thereof.
There have been known some constant current source circuits for providing a stable current to an electronic circuit, for example, as shown in FIGS. 1 and 2.
The constant current source circuit illustrated in FIG. 1 as a first conventional example is configured as follows.
The respective gates of transistors M1 and M2 each composed of an N-channel type MOSFET are connected to one another through a common connection, and the transistor M2 is short-circuited between the drain and gate thereof. The source of the transistor M1 is connected to GND via a resistor RS, and the source of the transistor M2 is connected directly to GND. The respective gates of transistors M3 and M4 each composed of a P-channel type MOSFET are connected to one another through a common connection, and the transistor M3 is short-circuited between the drain and gate thereof. Each of the sources of the transistors M3 and M4 is connected directly to a power supply point VCC.
The respective drains of the transistors M3 and M1 are connected to one another through a common connection, and the respective drains of the transistors M4 and M2 are connected to one another through a common connection. The transistors M1, M2, M3 and M4 and the resistor RS make up a self-biasing type constant current circuit 3b. 
The transistor M1 is designed to have a channel size (the ratio of channel width W/channel length L) N times greater than that of the transistor M2. While each of the channel sizes of the transistors M3 and M4 may be selectively set at an appropriate value in practice, given that the transistors M3 and M4 have the same channel size only for the sake of simplicity (given that transistors M3 and M4 illustrated in another circuit diagram also have the same channel size).
The constant current source circuit also includes an output transistor M6 composed of an N-channel type MOSFET, and the gate of the output transistor M6 is connected to the gate of the transistor M2 through a common connection. The source of the output transistor M6 is connected to GND, and the drain of the output transistor M6 is connected to a first output terminal 1. The constant current source circuit further includes an output transistor M7 composed of a P-channel type MOSFET, and the gate of the output transistor M7 is connected to the gate of the transistor M4 through a common connection. The source of the output transistor M7 is connected to the power supply point VCC, and the drain of the output transistor M7 is connected to a second output terminal 2.
The constant current source circuit in FIG. 1 operates as follows.
The transistors M3 and M4 are operable to allow a current to flow through the transistors M1 and M2 different in channel size, at the same value. Then, the transistors M1 and M2 operating in a cooperative manner under different current densities create a given voltage across the resistor RS. This voltage allow a highly stable current IR (hereinafter referred to as “stabilized current”) to flow through a line of the resistor RS and the transistors M1 and M3. Further, a reference current Iref equal to the stabilized current IR flows through a line of the transistors M2 and M4, and then an output current proportional to the reference current Iref(=stabilized current IR) is picked up from the output transistors M6 and M7.
The configuration and the operation of the circuit in FIG. 1 are disclosed, for example, in “Design Techniques of Analog Integrated Circuits (Vol. 1)” co-authored by P. R. Grey and P. G Meyer (pp 263–265, BAIFUKAN Co., Ltd., Dec. 15, 1990), “Design Techniques of Analog Integrated Circuits (Vol. 2)” co-authored by P. R. Grey and P. G. Meyer (pp 308–309, BAIFUKAN Co., Ltd., Dec. 15, 1990), and Japanese Patent Laid-Open Publication No. 08-228114 or 2002-116831.
The constant current source circuit illustrated in FIG. 2 as a second conventional example is configured as follows.
The gate of a transistor M8 composed of an N-channel type MOSFET is connected to the drain of a transistor M9 composed of an N-channel type MOSFET, and the source of the transistor M8 is connected to the gate of the transistor M9. The source of the transistor M9 is connected directly to GND, and the source of the transistor M8 is connected to GND via a resistor RS. The respective gates of transistors M3 and M4 each composed of a P-channel type MOSFET are connected to one another through a common connection, and the transistor M3 is short-circuited between the drain and gate thereof. Each of the sources of the transistors M3 and M4 is connected directly to a power supply point VCC.
The respective drains of the transistors M3 and M8 are connected to one another through a common connection, and the respective drains of the transistors M4 and M9 are connected to one another through a common connection. The transistors M3, M4, M8 and M9 and the resistor RS make up a self-biasing type constant current circuit 3c. 
The constant current source circuit in FIG. 2 operates as follows.
Based on the transistors M8 and M9 operating in a cooperative manner, a given voltage approximately equal to a threshold voltage Vth of the transistor M9 is created across the resistor RS. This voltage allows a stabilized current IR to flow through a line of the resistor RS and the transistors M8 and M3. Further, a reference current Iref equal to the stabilized current IR flows through a line of the transistors M9 and M4, and then an output current proportional to the reference current Iref(=stabilized current IR) is picked up from output transistors M6 and M7.
The configuration and the operation of the circuit in FIG. 2 are disclosed, for example, in “Design Techniques of Analog Integrated Circuits (Vol. 1)” co-authored by P. R. Grey and P. G. Meyer (pp 259–263, BAIFUKAN Co., Ltd., Dec. 15, 1990), “Design Techniques of Analog Integrated Circuits (Vol. 2)” co-authored by P. R. Grey and P. G. Meyer (pp 305–307, BAIFUKAN Co., Ltd., Dec. 15, 1990), and Japanese Patent Laid-Open Publication No. 2002-116831.
The constant current circuit having the configuration as shown in FIG. 1 or 2 is operable to stabilize the circuit operation on a self-biasing basis. As used in the specification, the term “self-biasing” means a feedback control characterized in that, based on a current mirror operation between the transistors M3 and M4, the reference current Iref flowing through the transistor M2 (or M9) is determined by the stabilized current IR flowing through the transistor M1 (or M8). Such a self-biasing type constant current source circuit has two operation points allowing the circuit to be stably operated thereat (hereinafter referred to as “stable operation points”). Specifically, the stable operation points consist of one point where “the stabilized current=zero” and the other point where “the stabilized current=a given current value”. In use of the circuit in FIG. 1 or 2, one of the stable operation points where the stabilized current=zero involves the following problem.
Firstly, even if a power is simply supplied from the current supply point VCC to activate the circuit, no current will flow through the transistors M1, M2, M3 and M4 (or M8, M9, M3 and M4). If nothing is done, the circuit in FIG. 1 (or FIG. 2) will never be activated.
Thus, it is typically required to additionally provide an activation circuit for injecting a current into a connection node P1 (FIG. 1) or P2 (FIG. 2) in the circuit. The activation circuit is essentially configured such that the current injection into the connection node P1 or P2 is discontinued just after activation of the constant current source circuit so as not to interfere with the normal operation of the constant current circuit 3b or 3c. 
Secondly, most of current electronic devices have a function of shifting a regular operation mode to another operation mode having a power consumption less than that in the regular operation mode (hereinafter referred to as “power saving operation mode”) if the device is not operated for a predetermined period of time. A constant current source circuit for use in such electronic devices is desirably designed to switchably control an output current in such a manner that it is increased in the regular operation mode, and reduced in the power saving operation mode. However, the circuit in FIG. 1 or 2 having only one stable operation point other than the point where the stabilized current=zero cannot vary the value of the output current if nothing is done. Thus, in order to vary the value of the output current, an additional control circuit for switching the output current is essentially provided between the output transistors M6, M7 and the constant current circuit 3b (or 3c) while bearing the disadvantage of complication in circuit configuration. One example of such a control circuit is disclosed, for example, in Japanese Patent Laid-Open Publication No. 08-241140.
The activation circuit or the control circuit additionally incorporated in the constant current circuit causes problems of the complication in circuit configuration and the increase in size of a semiconductor circuit board.
Further, while the resistor RS provided in the circuit in FIG. 1 or 2 may be set at a larger resistance value to reduce the output current of the circuit, the output current can be actually reduced to about several hundred nA only after the resistance value of the resistor RS is set at several MΩ. If a high resistance element of several MΩ is formed on a semiconductor circuit board, it will undesirably occupy a fairly large area on the board.